1. Field of the Invention
The present invention relates to the die attaching and wire bonding steps required to assemble a semiconductor package in the semiconductor and microcircuit manufacturing industry. The invention described integrates and simplifies the conventional steps taken during the manufacturing process.
2. Prior Art
Conventional methodology in the semiconductor and microcircuit industries commonly employs physically separate, complex, and automated machine systems for the die attaching step and for the wire bonding step in the assembly of a semiconductor package.
Present manufacturing technology accomplishes die attach in a number of different ways which include, but are not limited to the following:
1. Epoxy Paste--dispensed paste on a leadframe area is located beneath a die that is held by a surface contact tool or an edge contact only tool (collet). The die is lowered into the epoxy and is pressed down long enough to insure adhesion. X-Y movement (scrub) is sometimes used to increase adhesion and speed the process. This process requires a follow-on cure in a separate cure oven as shown in FIG. 1.
2. Epoxy Film--an epoxy film is dispensed on a leadframe area and the die is lowered down to the film surface. Bonding is accomplished with pressure. This process requires a follow-on cure in a separate cure oven as shown in FIG. 1.
3. Epoxy Film on Tape--an epoxy film that is applied to both sides of a supporting tape is dispensed on a leadframe area. Remaining steps are as in Epoxy Film, above.
4. Thermoplastic Film--a film with thermoplastic adhesive on both sides of the film is attached to the leadframe. The frame with the film attached is presented below the die and the die is attached using controlled force and temperature. Scrub of small magnitude may be used. This method does not require follow-up cure.
5. Thermoplastic Tape--a piece of thermoplastic material is attached and bonded as in the Thermoplastic Film method.
6. Eutectic--metal with a low melting temperature is dispensed onto the leadframe area. A die is placed on the dispensed metal. Adhesion is obtained by an intermixing of the die backside and the metal. Controlled pressure, scrub, and temperature are used. No follow-on cure is required.
7. Soft Solder--same process as in Eutectic except that the metal does not mix with the backside material.
8. Glue--a conductive or non-conductive glue can be used as required. The glue would normally be quick set with no follow-on cure required.
A relevant and now common method, area wire bond (A-wire), will be described. The A-wire process, which complements mechanical, thermal, and electrical device performance functions, is applied to surface mount multiple lead integrated circuits to form an internal plastic package construction. Typical discussions of current IBM area wire bond techniques may be found in U.S. Pat. No. 4,862,245, William C. Ward, et al., and in an an article in IEEE Journal, Mar., 1988, pp 552-557, by William C. Ward, entitled, in part, "Volume Production of Unique Plastic Surface-mount Modules . . . by area Wire Bond Techniques".
Briefly, in the conventional IBM A-wire process two highly automated machines are used. First, a die attach machine lays down a thermoset film, which has a heat-activated adhesive on each side, to the central area of a lead frame. A die is then bonded to the film in a way which allows wires to be attached to bond pads on the die which are exposed by "via-holes" in the film. The part is then moved to an oven for tape cure. After tape cure the material is transferred to the wirebonder machine where fine wires are attached which connect the die pads to the individual leads on the frame. The sequence is described in more detail immediately below:
1. A lead frame is inserted into the transport track of the die attacher.
2. The leadframe is moved or "indexed" to a location where an accurately cut piece of thermoset film is attached precisely to the bottom side of the lead frame. The film is attached using a thermode for the controlled application of heat and pressure.
3. The lead frame, with the film firmly laminated to the bottom of the leads, is indexed into position for the die bonding.
4. A die is mechanically picked out of a tray known as a "waffle pack" and is precisely positioned below the film bonded area of the leads on the lead frame. The die attach system must be able to move the die in the X, Y, and Z and .THETA. (Theta) axes with a high control accuracy.
5. When the die is successfully registered, another thermode is used to attach the die to the film by the controlled use of heat and mechanical force. A die is successfully registered when the circuitry on the face of the die aligns with the film to cover the memory array and provide access for bonding through "via-holes".
6. The lead frame is indexed to a bonding position known as the post die attach lamination station. Another thermode securely laminates all of the elements together, effectively creating a single, uniformly-structured package. The heat and force are controlled as before, but with a different profile than used previously.
7. The part is then removed and placed in an oven to cure the tape.
8. The completed part or package is then indexed through the remaining track system and into a carrier for transfer to the wire bonder. Throughout the die attach process, all positioning or indexing movements are mechanical. Precision of the indexing movements is monitored by random visual inspection of the various steps.
9. The carrier containing the lead frames from the die attach machine is transferred manually to the wirebonder for attachment of the fine wires to the leads as shown in FIG. 2.
10. The individual lead frames are indexed into the track of the wirebonder where they are clamped in the bonding position. Wires are then automatically attached from each die pad, in turn, to each lead of the lead frame.
11. Next, the lead frame is indexed out through the track and into another carrier. This carrier is moved off the wirebonder to the next process step or to storage.
To achieve the needed positional accuracy prior to wirebonding, a computer controlled television camera (CCTV), which is attached to the moving bond head, takes a picture of two different areas of the die metallization. This information is transferred to the pattern recognition system, which is a subsystem of the wirebonder. By use of digital circuitry, the actual position of the pattern on the die is established relative to the lead frame.
There is a displacement or "offset" between the capillary position, which contains the wire to be attached, and the line-of-sight needed by the camera to view the die pattern. At each index, when a new location is presented for the bonding operation, the bond head must move to a location that allows the camera to have a clear view of the die pattern. This distance approximates 0.500".
In conventional die attach and wire bond machines the multiple lead frames on the carrier are advanced in a horizontal plane. This is accomplished with multiple index rollers, which rotate continuously about axes above and at right angles to, and in parallel relationship with, the movable horizontal surfaces of the lead frame carriers. The rollers rotate in a direction opposite of the direction of travel of the lead frame carriers. Movement of the carriers is started and stopped by lowering and raising the rollers mechanically to make or release contact with the carriers. When a carrier is stopped in a pre-selected position it is indexed precisely by guide pins located below the horizontal plane of the carrier. When a command is given, the pins move up and engage with the matching holes in the lead frame carrier. The head of the guide pin is generally conical, while the smaller hole in the carrier that matches the guide pin is generally round. Size differences between the pins and holes in the die attach and wire bond machines often allow positional variations of .+-.0.003" in a particular index location.
Once the wirebond machine starts to work on a die, it will attempt to bond wire to all the die pads which have been programmed into the machine. Modern systems have means to sense many rejectable faults, such as "non-stick" and "no wire". However, some faults can only be detected by visual inspection, such as when ball bonds are partially off the pads.
As mentioned above, some wirebond machines have a CCTV camera attached to the bond head to observe and control the bonding step. Commercial CCTV cameras and monitors are available that can be mounted separately from the bond head. This separate mounting allows the cameras to produce a large image of the part but not be affected by the movement of the bond head. It also increases the accuracy of determining the position of the parts.
It is an intent of this invention to integrate these normally disparate processes into both the die attach and wirebond processes. Integrating these processes would provide real-time, useful feedback from each process step to the previous and to the following step, thereby improving positioning accuracy and precision while reducing rejectable faults. Additionally, by tying the processes tightly together, the wirebond process validates the successful completion of the die attach, and the optical inspection validates the wirebond step. By using a more sophisticated vision system, information gathered at one process step is available to apply to the next, without the usual problems associated with machine-to-machine manual transfer, communication, or data conversion.
By mounting the vision system statically rather than attaching it to the moving bond head, a significant improvement in accuracy and time savings can be realized at the start of the bond cycle. Feeding the information sensed by the vision system to or from the die attach and wire bonder control systems yields an adaptive, self-regulating assembly system through integration of visual process monitoring into process control.
It is a further intent of this invention to reduce the number of indexing steps required in the die attach and A-wire bond processes in order to increase the overall quality and quantity of the product and to reduce time and cost of manufacture.